ZF 9HP

ZF's nine-speed automatic, and the one that had to be small: it was the first nine-speed built for a transverse engine, which means it had to fit the space a six-speed occupied in a front-wheel-drive car. It does, almost exactly — about six millimetres longer than the unit it replaced and some seven kilograms lighter, out of four gearsets, three brakes and three clutches.

It is not the 8HP, and the numbers being adjacent is the whole problem. The ZF 8HP is a longitudinal gearbox for rear- and all-wheel-drive cars; this one is transverse, for front-drive-based ones. They share a maker and a naming scheme and nothing else — no parts, no applications, no failure modes. A BMW and a Jeep Cherokee do not have nearly the same gearbox.

Two patented dog clutches are what made it possible, and they are also why it arrived with the reputation it did. A dog clutch engages positively — teeth into teeth — instead of squeezing friction plates together, which is how ZF saved the bulk that nine ratios would otherwise have cost. The trade-off is that a dog clutch has no slip to hide behind. A friction pack can be feathered to smooth a change; a dog clutch is either in or out, and getting that to feel like an automatic is a control problem rather than a hardware one.

Which is exactly how it went wrong, and exactly why the fix was what it was. Early FCA cars — the 2014 Jeep Cherokee above all — shipped with harsh upshifts and downshifts, hesitant engagement, and a run of ratio-error and stuck-in-gear fault codes. ZF's position was that these were software rather than mechanical faults, and the record supports it: Chrysler issued service bulletins to recalibrate, Acura recalled the 2015 TLX, and a great many of these cars were genuinely fixed by reprogramming the control module.

That makes it the opposite of most gearboxes on this site, and it is worth saying plainly. On an Aisin TF-80SC with worn valve bores, no amount of reprogramming will compensate for a hydraulic leak, and a shop offering software is selling you nothing. On a 9HP with a shift-quality complaint, the calibration really is the most likely answer — so the first move is to check for outstanding bulletins and recalls against the vehicle, before anybody opens anything.

What fails mechanically, once past that:

• The mechatronic unit, which is the commonest hardware failure and is electronic rather than hydraulic.
• Heat damage from sustained running, which is the underlying cause of premature wear through the unit.
• A silted heat exchanger, and here the trade's finding is worth repeating: cleaning it is not reliably enough on a high-mileage unit, because by then the wear it caused has already happened.

The practical summary for an owner: a rough-shifting 9HP is a car to check the service bulletins on, not a car to condemn. It is a genuinely clever design that was released before its software was ready, and the two things that follow from that are that early cars behaved badly and that most of them were fixable without parts.